U.S. patent number 9,383,573 [Application Number 14/376,423] was granted by the patent office on 2016-07-05 for phase modulation device and laser microscope.
This patent grant is currently assigned to CITIZEN HOLDINGS CO., LTD.. The grantee listed for this patent is Nobuyuki Hashimoto, Makoto Kurihara, Kenji Matsumoto, Ayano Tanabe, Masafumi Yokoyama. Invention is credited to Nobuyuki Hashimoto, Makoto Kurihara, Kenji Matsumoto, Ayano Tanabe, Masafumi Yokoyama.
United States Patent |
9,383,573 |
Matsumoto , et al. |
July 5, 2016 |
Phase modulation device and laser microscope
Abstract
A phase modulation device corrects wave front aberrations
generated by an optical system including an objective lens disposed
on an optical path of a light flux. The phase modulation device
includes a phase modulation element which includes a plurality of
electrodes, and modulates the phase of the light flux in accordance
with a voltage applied to each electrode, and a control circuit
which controls the voltage to be applied to each electrode. The
control circuit controls the voltage to be applied to each
electrode in such a manner that the light flux is imparted with a
phase modulation amount in accordance with a phase modulation
profile having a polarity opposite to the polarity of a phase
distribution to be determined according to a relational equation
representing a relationship between a numerical aperture of the
objective lens and a ratio between third-order spherical aberration
and fifth-order spherical aberration.
Inventors: |
Matsumoto; Kenji (Tokyo,
JP), Tanabe; Ayano (Tokyo, JP), Yokoyama;
Masafumi (Tokyo, JP), Hashimoto; Nobuyuki
(Saitama, JP), Kurihara; Makoto (Saitama,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Matsumoto; Kenji
Tanabe; Ayano
Yokoyama; Masafumi
Hashimoto; Nobuyuki
Kurihara; Makoto |
Tokyo
Tokyo
Tokyo
Saitama
Saitama |
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP |
|
|
Assignee: |
CITIZEN HOLDINGS CO., LTD.
(Tokyo, JP)
|
Family
ID: |
48905410 |
Appl.
No.: |
14/376,423 |
Filed: |
February 1, 2013 |
PCT
Filed: |
February 01, 2013 |
PCT No.: |
PCT/JP2013/052390 |
371(c)(1),(2),(4) Date: |
August 02, 2014 |
PCT
Pub. No.: |
WO2013/115383 |
PCT
Pub. Date: |
August 08, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150338639 A1 |
Nov 26, 2015 |
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Foreign Application Priority Data
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Feb 3, 2012 [JP] |
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2012-021665 |
Jul 4, 2012 [JP] |
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2012-150194 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
21/0052 (20130101); G02B 21/0056 (20130101); G02B
21/0032 (20130101); G02B 21/082 (20130101); G02B
27/0068 (20130101); G02B 26/06 (20130101); G02F
2203/18 (20130101); G02F 1/134309 (20130101); G02F
2203/50 (20130101) |
Current International
Class: |
G02B
26/06 (20060101); G02B 21/08 (20060101); G02B
21/00 (20060101); G02B 27/00 (20060101); G02F
1/1343 (20060101) |
Field of
Search: |
;359/237,238,240,245,279 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3299808 |
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Jul 2002 |
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JP |
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2005-224328 |
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Aug 2005 |
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JP |
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2005-267756 |
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Sep 2005 |
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JP |
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2006-330089 |
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Dec 2006 |
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JP |
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2007-134023 |
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May 2007 |
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JP |
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4149309 |
|
Sep 2008 |
|
JP |
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2011/105618 |
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Sep 2011 |
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WO |
|
Other References
David A. Horsley ; Hyunkyu Park ; Sophie P. Laut and John S. Werner
"Characterization for vision science applications of a bimorph
deformable mirror using phase-shifting interferometry", Proc. SPIE
5688, Ophthalmic Technologies XV, 133 (May 2, 2005);
doi:10.1117/12.591848; http://dx.doi.org/10.1117/12.591848. cited
by examiner .
International Search Report for PCT Application No.
PCT/JP2013/052390, Apr. 23, 2013. cited by applicant .
International Written Opinion for PCT Application No.
PCT/JP2013/052390, Apr. 23, 2013. cited by applicant .
European Patent Office, Extended European Search Report for EP
Patent Application No. 13743360.3, Aug. 5, 2015. cited by
applicant.
|
Primary Examiner: Won; Bumsuk
Assistant Examiner: Alexander; William R
Claims
What is claimed is:
1. A phase modulation device for correcting wave front aberrations
generated by an optical system including an objective lens disposed
on an optical path of a light flux of coherent light to be emitted
from a coherent light source, comprising: a phase modulation
element which comprises a plurality of annular electrodes in a
concentric form, the center of which is an optical axis of the
optical system, and modulates a phase of the light flux
transmitting through the objective lens in accordance with a
voltage applied to each of the annular electrodes; and a control
circuit which controls the voltage to be applied to each of the
plurality of annular electrodes, wherein the control circuit
controls the voltage to be applied to each of the plurality of
annular electrodes in such a manner that the light flux is imparted
with a phase modulation amount in accordance with a phase
modulation profile which has a polarity opposite to a polarity of a
phase distribution of the wave front aberrations and is determined
according to a relational equation representing a relationship
between a numerical aperture of the objective lens and a ratio
between third-order spherical aberration and fifth-order spherical
aberration when the phase distribution of the wave front
aberrations is resolved using Zernike polynomials; and the control
circuit applies a voltage to one of the annular electrodes
corresponding to a first position where the phase modulation amount
of the phase modulation profile is maximum in such a manner that a
maximum value of the phase modification amount is generated on the
first position and applies a voltage to another one of the annular
electrodes corresponding to a second position where the phase
modulation amount of the phase modulation profile is minimum in
such a manner that a minimum value of the phase modulation amount
is generated on the second position.
2. The phase modulation device according to claim 1, wherein the
objective lens is an immersion objective lens, and the numerical
aperture of the objective lens satisfies the following condition:
1.15.ltoreq.NA.ltoreq.1.27 where NA represents the numerical
aperture of the objective lens, and the relational equation is
represented by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00008## where A represents a
third-order spherical aberration component, B represents a
fifth-order spherical aberration component, and NA represents the
numerical aperture of the objective lens.
3. The phase modulation device according to claim 1, wherein the
objective lens is an immersion objective lens, and the numerical
aperture of the objective lens satisfies the following condition:
1.05.ltoreq.NA.ltoreq.1.27 where NA represents the numerical
aperture of the objective lens, and the relational equation is
represented by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00009## where A represents a
third-order spherical aberration component, B represents a
fifth-order spherical aberration component, and NA represents the
numerical aperture of the objective lens.
4. The phase modulation device according to claim 1, wherein the
objective lens is a dry objective lens, and the numerical aperture
of the objective lens satisfies the following condition:
0.75.ltoreq.NA.ltoreq.0.95 where NA represents the numerical
aperture of the objective lens, and the relational equation is
represented by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00010## where A represents a
third-order spherical aberration component, B represents a
fifth-order spherical aberration component, and NA represents the
numerical aperture of the objective lens.
5. The phase modulation device according to claim 1, wherein the
phase modulation profile is determined in such a manner that a
phase modulation amount on an optical axis of the optical system is
equal to a phase modulation amount at an end of an active region,
the active region being a region capable of phase-modulating a
light flux on the phase modulation element.
6. The phase modulation device according to claim 1, wherein the
phase modulation profile is determined in such a manner that a root
mean square value of the phase modulation profile is minimized.
7. The phase modulation device according to claim 1, wherein the
phase modulation element is a liquid crystal element.
8. The phase modulation device according to claim 7, wherein the
control circuit adjusts the phase modulation profile in accordance
with a wavelength of the coherent light.
9. The phase modulation device according to claim 1, wherein the
annular electrodes are connected to each other by one or more
resistors, and a voltage applied to each of the annular electrodes
corresponding to a position other than the first and second
positions being determined by dividing a difference between the
voltage to be applied to the annular electrode whose phase
modulation amount is maximum, and the voltage to be applied to the
annular electrode whose phase modulation amount is minimum, by a
resistance value of corresponding resistor connected between the
annular electrodes.
10. The phase modulation device according to claim 1, wherein the
control circuit applies a voltage to an outermost peripheral
annular electrode, as well as to the annular electrodes
corresponding to the first position where the phase modulation
amount is maximum and corresponding to the second position where
the phase modulation amount is minimum.
11. A laser microscope, comprising: a coherent light source which
irradiates coherent light; a first optical system disposed on an
optical path of a light flux of the coherent light, and including
an objective lens to focus the light flux on a specimen; a second
optical system which transmits a light flux including specimen
information derived from the specimen to a detector; and the phase
modulation device of claim 1, wherein the phase modulation element
of the phase modulation device is disposed between the coherent
light source and the objective lens.
Description
TECHNICAL FIELD
The present invention relates to a technology, in an apparatus
including an objective lens and using a coherent light source, in
which a light flux to be irradiated on a specimen is
phase-modulated, and aberrations generated depending on the
specimen or various conditions are compensated for acquiring
information having enhanced resolution.
BACKGROUND ART
A confocal laser microscope is configured such that laser light is
focused on a specimen through an objective lens, a light flux of
reflected light, scattered light, or fluorescent light generated on
the specimen is transmitted by an optical system, and the light
flux transmitted through a pinhole disposed at an optically
conjugated position with respect to a light focusing point on the
specimen is received on a detector. Disposing the pinhole makes it
possible to filter the light generated on the specimen other than
the light focusing point. Therefore, the confocal laser microscope
is operable to acquire an image with a good S/N ratio.
Further, the confocal laser microscope is configured to acquire a
planar image of a specimen by scanning the specimen with laser
light along two directions (X-direction and Y-direction) orthogonal
to each other, along a plane perpendicular to the optical axis. On
the other hand, the confocal laser microscope is configured to
acquire a plurality of tomographic images (Z-stack images) in
Z-direction by changing the distance in the optical axis direction
(Z-direction) between the objective lens and the specimen, whereby
a three-dimensional image of the specimen is formed.
In observing a biospecimen, it is often the case that the
biospecimen is observed through a cover glass in a state in which
the biospecimen is immersed in a broth. Further, generally, the
objective lens is designed so that an optimum imaging performance
at a position immediately below the cover glass is best. In
observing the inside of a biospecimen, it is necessary to acquire
an image transmitted through a broth or biological tissues and
having a certain depth at an observation position. Aberrations are
generated in proportion to the distance from the position
immediately below the cover glass to the observation position, and
as a result, the resolution may be lowered.
Further, the cover glasses have variations in the thickness thereof
within the tolerance range from the design value (e.g. 0.17 mm).
Aberrations are generated in proportion to a difference between the
actual thickness of the cover glass and the design thickness due to
a difference between the refractive index (=1.525) of the cover
glass and the refractive index (=1.38 to 1.39) of the biospecimen.
Further, when the objective lens is an immersion lens, aberrations
are generated in proportion to the depth of a biospecimen with
respect to the observation position due to a difference between the
refractive index of the biospecimen and the refractive index
(=1.333) of water in the same manner as described above. As a
result, the resolution to be obtained in observing a deep part of
the biospecimen may be lowered.
As one means for solving the above defects, a correction ring has
been proposed. The correction ring is a ring-shaped rotating member
provided in an objective lens. The distances between lens groups
constituting the objective lens is changed by rotating the
correction ring. Aberrations due to an error in the thickness of
the cover glass or observing a deep part of the biospecimen are
cancelled by rotating the correction ring. A scale is marked on the
correction ring. For instance, rough numerical values such as 0,
0.17, and 0.23 are indicated concerning the thickness of the cover
glass. Adjusting the scale of the correction ring in accordance
with the thickness of an actually used cover glass makes it
possible to adjust the distances of the lenses in such a manner as
to optimize the distances in accordance with the thickness of the
cover glass (e.g. see Patent Literature 1).
Further, there is also known a technique of compensating for
generated aberrations by a wave front conversion element. This
technique is a matrix-drivable shape variable mirror element that
is disposed on an optical path of a microscope, a wave front is
modulated by the shape variable mirror element based on wave front
conversion data measured in advance, and the modulated light wave
is allowed to be incident on a specimen, whereby an
aberration-corrected image with a high imaging performance is
acquired (see e.g. Patent Literature 2).
As the wave front conversion element, a shape variable mirror
element configured such that the shape of a reflection surface
thereof is electrically controllable is used. When a plane wave is
incident on the shape variable mirror element, and if the shape
variable mirror element has a concave shape, the incident plane
wave is converted into a concave wave front (the amplitude of a
concave shape is doubled).
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Publication No. 3,299,808 (see
pages 4-6, and FIG. 1)
Patent Literature 2: Japanese Patent Publication No. 4,149,309 (see
pages 3-5, and FIG. 1)
SUMMARY OF INVENTION
Technical Problem
However, the operation of the correction ring is performed by
manually rotating a ring-shaped adjustment mechanism provided on
the objective lens. Therefore, focus deviation or view field
deviation resulting from adjusting the adjustment mechanism may
occur. Further, it is necessary to repeat adjusting the correction
ring and focusing in order to determine the optimum position of the
objective lens. This may make the process for optimization
cumbersome. Since the process is cumbersome, it takes time to make
adjustments in order to obtain an optimum position, and a
fluorescent pigment may fade. Fading of a fluorescent pigment may
weaken the fluorescent intensity due to continuous emission of
excitation light.
Further, adjustment of the correction ring requires fine control.
Under the present circumstances, judgment on the adjustment result
relies on a person who visually observes an image. It is very
difficult to judge whether the objective lens is located at an
optimum position. In particular, in photographing images of Z
stack, it is necessary to repeat the above operation by the number
of times equal to the number of images to be acquired in depth
direction, which is very cumbersome. As a result, under the present
circumstances, the number of users who sufficiently utilize the
correction ring may be small. Further, in some specimens,
vibrations resulting from touching the correction ring by hand may
affect the observation position. In view of the above, it is
desirable to automatically adjust the correction ring without
touching the correction ring by hand.
Further, in the technology of compensating for aberrations by a
wave front conversion element, the optical system of a microscope
may be complicated and the size of the optical system may increase,
because the wave front conversion element is of a reflective type.
Furthermore, it is necessary to measure the aberrations in advance
in order to obtain an optimum compensated wave front. A process of
converging the correction amount in order to form an optimum wave
front is required. Therefore, this technology is less feasible.
In view of the above, an object of the invention is to solve the
above problems and to provide a phase modulation device that
corrects for aberrations generated depending on a specimen or an
observation condition, without the need of a drastic change in an
existing optical system and without the need of touching an
objective lens by hand. Another object of the invention is to
provide a laser microscope incorporated with the phase modulation
device that enables acquiring an image having a high imaging
performance.
Solution to Problem
In order to solve the above drawbacks and to accomplish the
objects, a phase modulation device for correcting wave front
aberrations generated by an optical system including an objective
lens disposed on an optical path of a light flux of coherent light
to be emitted from a coherent light source has the following
configuration.
The phase modulation device includes a phase modulation element
which includes a plurality of electrodes, and modulates a phase of
the light flux transmitting through the objective lens in
accordance with a voltage applied to each of the electrodes; and a
control circuit which controls the voltage to be applied to each of
the electrodes. The control circuit controls the voltage to be
applied to each of the electrodes in such a manner that the light
flux is imparted with a phase modulation amount in accordance with
a phase modulation profile having a polarity opposite to a polarity
of a phase distribution of the wave front aberrations to be
determined according to a relational equation representing a
relationship between a numerical aperture of the objective lens and
a ratio between third-order spherical aberration and fifth-order
spherical aberration when the phase distribution of the wave front
aberrations generated by the optical system is resolved using
Zernike polynomials.
In the phase modulation device, preferably, the relational equation
representing the relationship between the numerical aperture of the
objective lens and the ratio between third-order spherical
aberration and fifth-order spherical aberration may be represented
by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00001##
Alternatively, in the phase modulation device, preferably, the
relational equation representing the relationship between the
numerical aperture of the objective lens and the ratio between
third-order spherical aberration and fifth-order spherical
aberration may be represented by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00002##
Further alternatively, in the phase modulation device, preferably,
the relational equation representing the relationship between the
numerical aperture of the objective lens and the ratio between
third-order spherical aberration and fifth-order spherical
aberration may be represented by the following equation:
.times..ltoreq..ltoreq..times. ##EQU00003##
Note that, in the relational equations described above, A
represents a third-order spherical aberration component, B
represents a fifth-order spherical aberration component, and NA
represents a numerical aperture of the objective lens.
Further, in the phase modulation device, preferably, the phase
modulation profile may be determined in such a manner that a phase
modulation amount on an optical axis is equal to a phase modulation
amount at an end of an active region, the active region being a
region capable of phase-modulating a light flux on the phase
modulation element.
Alternatively, in the phase modulation device, preferably, the
phase modulation profile may be determined in such a manner that a
root mean square value of the phase modulation profile is
minimized.
Further, in the phase modulation device, preferably, the phase
modulation element may be a liquid crystal element.
Further, in the phase modulation device, preferably, the control
circuit may adjust the phase modulation profile in accordance with
a wavelength of the coherent light.
Further, in the phase modulation device, preferably, the electrodes
may include a plurality of annular electrodes in a concentric form,
the center of which is an optical axis.
Further, in the phase modulation device, preferably, the annular
electrodes may be connected to each other by one or more resistors,
and the control circuit may apply a voltage to the annular
electrode corresponding to a first position where the phase
modulation amount of the phase modulation profile is maximum so
that a maximum value of the phase modulation amount is generated on
the first position and applies a voltage to the annular electrode
corresponding to a second position where the phase modulation
amount of the phase modulation profile is minimum in such a manner
that a minimum value of the phase modulation amount is generated on
the second position. A voltage applied to each of the annular
electrodes corresponding to a position other than the first and
second positions is determined by dividing a difference between the
voltage to be applied to the annular electrode whose phase
modulation amount is maximum, and the voltage to be applied to the
annular electrode whose phase modulation amount is minimum, by a
resistance value of corresponding resistor connected between the
annular electrodes.
Further, in the phase modulation device, preferably, the control
circuit may apply a voltage to an outermost peripheral annular
electrode, as well as to the annular electrodes corresponding to
the first position where the phase modulation amount is maximum and
corresponding to the second position where the phase modulation
amount is minimum.
Further, according to another aspect of the invention, a laser
microscope is provided. The laser microscope includes: a coherent
light source which irradiates coherent light; a first optical
system disposed on an optical path of a light flux of the coherent
light, and including an objective lens to focus the light flux on a
specimen; a second optical system which transmits a light flux
including specimen information derived from the specimen to a
detector; and the phase modulation device having one of the above
configurations. The phase modulation element of the phase
modulation device is disposed between the coherent light source and
the objective lens.
Advantageous Effects of Invention
According to the invention, a phase modulation device, and a laser
microscope incorporated with the phase modulation device are
capable of compensating aberrations generated by deviation of the
thickness of a cover glass from the design value, when a deep part
of a biospecimen is observed, or when the specimen is observed
through the cover glass, and observing the specimen with enhanced
resolution. In particular, the phase modulation device is operable
to electrically compensate for aberrations without the need of
touching a lens by hand. Therefore, it is possible to eliminate the
cumbersomeness such as adjusting a correction ring. Thus, the phase
modulation device and the laser microscope are advantageous in
automatically optimizing the position of the objective lens and in
adjusting the position of the objective lens in synchronization
with the observation depth in the Z stacking process. Further, the
phase modulation device and the laser microscope are capable of
minimizing the phase correction amount for adjustment. Furthermore,
it is possible to correct aberrations of objective lenses having
numerical apertures NAs different from each other by one phase
modulation device.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic configuration diagram of a laser microscope
according to one embodiment of the invention;
FIG. 2A is a diagram for representing aberrations generated in
observing the surface of a specimen and in observing the inside of
the specimen at the depth D;
FIG. 2B is a diagram for representing aberrations generated in
observing the surface of a specimen and in observing the inside of
the specimen at the depth D;
FIG. 3A is a diagram illustrating a phase distribution of
third-order spherical aberration;
FIG. 3B is a diagram illustrating a phase distribution of
fifth-order spherical aberration;
FIG. 4 is a diagram illustrating a sectional profile of a phase
distribution of complex aberrations that minimizes the RMS
value;
FIG. 5 is a diagram illustrating a sectional profile of a phase
distribution of complex aberrations that minimizes the PV value of
the phase modulation amount;
FIG. 6 is a schematic diagram for representing a phase modulation
device for use in the laser microscope according to one embodiment
of the invention;
FIG. 7 is a sectional schematic view of a liquid crystal device
with homogeneous alignment that serves as a phase modulation
device;
FIG. 8 is a diagram for representing a method for determining an
annular electrode structure of a phase modulation device according
to one embodiment of the invention;
FIG. 9A is a schematic diagram representing a method for connecting
between annular electrodes and a method for applying voltages to
annular electrodes in the phase modulation device according to one
embodiment of the invention;
FIG. 9B is a schematic diagram representing a method for connecting
between annular electrodes and a method for applying voltages to
annular electrodes in the phase modulation device according to one
embodiment of the invention;
FIG. 10A is a diagram illustrating a relationship between the
numerical aperture NA and the ratio (Z.sub.25/Z.sub.13) when the
numerical aperture NA of the objective lens is in the range of from
1.15 to 1.27, and the objective lens is an immersion lens;
FIG. 10B is a diagram illustrating a relationship between the
numerical aperture NA and the ratio (Z.sub.25/Z.sub.13) when the
numerical aperture NA of the objective lens is in the range of from
1.05 to 1.27, and the objective lens is an immersion lens;
FIG. 10C is a diagram illustrating a relationship between the
numerical aperture NA and the ratio (Z.sub.25/Z.sub.13) when the
numerical aperture NA of the objective lens is in the range of from
0.75 to 0.95, and the objective lens is a dry lens;
FIG. 11A is a diagram illustrating a sectional profile of a phase
distribution of spherical aberration generated by an objective lens
whose numerical aperture NA is 1.0;
FIG. 11B is a diagram illustrating a sectional profile of a phase
distribution of spherical aberration generated by an objective lens
whose numerical aperture NA is 1.2;
FIG. 11C is a diagram illustrating a sectional profile of a phase
distribution of spherical aberration generated by an objective lens
whose numerical aperture NA is 0.8; and
FIG. 12 is a diagram illustrating wavelength dispersion of a liquid
crystal device with respect to laser light wavelength.
DESCRIPTION OF EMBODIMENTS
In the following, preferred embodiments of a phase modulation
device and a laser microscope incorporated with the phase
modulation device according to the invention are described in
details referring to the drawings.
FIG. 1 is a schematic configuration diagram of a laser microscope
according to one embodiment of the invention. A light flux emitted
from a laser light source 1 as a coherent light source is adjusted
by a collimating optical system 2 into parallel light beam. After
the parallel light beam is transmitted through a phase modulation
device 3, the modulated light is focused on a specimen 5 through an
objective lens 4. A light flux including specimen information such
as a light flux reflected or scattered on the specimen 5 or
fluorescent light generated on the specimen returns along an
optical path, is reflected on a beam splitter 6, and is focused
again on a confocal pinhole 8 by a confocal optical system 7 as a
second optical system. The confocal pinhole 8 cuts a light flux on
the specimen at a position other than the focus position.
Therefore, it is possible to obtain a signal having a good S/N
ratio by a detector 9.
The objective lens 4 is designed taking into consideration
parameters including not only the inside of a lens system, but also
the refractive index and the length of the optical path from a lens
tip to an observation plane, for instance, the thickness of a cover
glass or the presence or absence of a cover glass, and in such a
manner that the imaging performance of the objective lens is
optimized under the condition with estimated values of these
parameters. According to the above configuration, aberrations may
be generated due to the depth of a biospecimen as an object to be
observed, or a thickness deviation resulting from manufacturing
error of a cover glass. The aberrations may lower the imaging
performance. In view of the above, the laser microscope is
configured to enhance the imaging performance by estimating wave
front aberrations generated by an optical system from the laser
light source 1 to the light focusing position of a light flux,
including the objective lens 4, in accordance with the deviation of
the optical path length from the design value; and by displaying,
on the phase modulation device 3, a phase distribution that cancels
the wave front aberrations as a phase modulation profile.
Generally, it is not possible to dispose a phase modulation device
at a pupil position of an objective lens, in view of the space.
Therefore, the phase modulation device 3 is disposed at a position
conjugate to the pupil, with use of a relay lens. Further, a light
flux emitted from the laser light source 1 passes through the phase
modulation device 3 twice along an outward path and along a return
path. Therefore, the phase modulation device 3 corrects the phase
of the light flux along the outward path and along the return path.
On the other hand, generally, an objective lens in a microscope is
designed to be an infinite system, and a light flux incident on the
objective lens is a parallel light beam. In view of the above, it
is preferable to dispose the phase modulation device 3 on the light
source side of the objective lens 4, specifically, at a position in
the vicinity of the objective lens 4. Disposing the phase
modulation device 3 as described above is advantageous for the
laser microscope to effectively obtain the correction effects.
Aberrations which may be generated are described in details. FIG.
2A and FIG. 2B are diagrams schematically illustrating aberrations
generated depending on the depth of a specimen to be observed. To
simplify the description, the objective lens is designed to be
optimized in observing a medium having a uniform refractive index.
FIG. 2A illustrates a light flux 200 in observing a medium having a
uniform refractive index, as used in the design. FIG. 2A
illustrates that the light flux 200 is focused on one point without
aberrations. Contrary to the above, FIG. 2B illustrates a light
flux 210 in observing the surface of a specimen at the depth D. The
light flux 210 is refracted on an interface 211 between the medium
in contact with the objective lens and the specimen. The light flux
210 is not focused on one point due to the generated
aberrations.
As described above, aberrations are not generated when observing
the surface of a specimen, but are generated when observing the
inside of the specimen. The laser microscope is generates a phase
distribution that cancels wave front aberrations, assuming that the
generated aberrations are represented as the wave front aberrations
at the pupil position of the objective lens 4, by applying voltages
to electrodes of the phase modulation device 3 disposed at the
pupil position of the objective lens. According to this
configuration, the laser microscope is operable to focus a light
flux from the laser light source 1 on one point at an observation
position defined on the surface of a specimen or in the inside of
the specimen. A light flux generated on a specimen also returns
along the optical path in the same manner as described above.
Therefore, the laser microscope is operable to convert the light
flux into a plane wave.
Wave front aberrations can be represented as a sum of components by
resolving the aberrations into the components. It is common to
resolve wave front aberrations into orthogonal functions such as
Zernike polynomials, and represent the wave front aberrations as a
sum of the functions. In view of the above, there is supposed a
method for obtaining a correction amount for wave front aberrations
by representing the correction amount as a phase distribution of
each of the functions of Zernike polynomials, and by changing the
relative phase modulation amount of each of the functions. For
instance, when aberrations are resolved using the standard Zernike
polynomials, the 13-th coefficient (Z.sub.13) represents
third-order spherical aberration, and 25-th coefficient (Z.sub.25)
represents fifth-order spherical aberration. Appropriately
adjusting the phase distribution of a correction amount
corresponding to each of the coefficients allows for the phase
modulation device 3 to correct the third-order spherical aberration
and the fifth-order spherical aberration.
Aberrations generated in observing a deep part of a specimen are
complex aberrations as combination of defocus or lower-order
spherical aberrations and higher-order spherical aberrations. For
instance, even if the phase modulation device 3 corrects Z.sub.13,
improvement of the imaging performance is not sufficient. Further,
Zernike polynomials are constituted of multitudes of terms.
Therefore, it is necessary to create a phase modulation profile
corresponding to each term, and to cause the phase modulation
device 3 to display the phase modulation profiles in order to
perform fine correction. In view of the above, it is preferable to
dispose an element obtained by placing a plurality of aberration
correction elements one over the other in a light flux, and to use
at least one of the aberration correction elements so as to display
the plurality of phase modulation profiles.
Actually, however, defocus sensitively changes depending on the
depth Z of a specimen. Therefore, defocus is determined by the
observation position of the specimen. Further, it is possible to
neglect aberrations other than Z.sub.13 and Z.sub.25 in Zernike
polynomials, because these aberrations are very small. Thus, it is
possible to enhance the imaging performance by correcting the term
Z.sub.13 corresponding to third-order spherical aberration and the
term Z.sub.25 corresponding to fifth-order spherical aberration.
Further, it is possible to sufficiently and satisfactorily correct
aberrations by taking into consideration defocus, third-order
spherical aberration, and fifth-order spherical aberration, and
even seventh-order spherical aberration in some cases. Thus, taking
into consideration disadvantages generated by placing a plurality
of aberration correction elements one over the other, for instance,
lowering of transmittance resulting from reflection on the
interfaces between the aberration correction elements, it is not
necessary to correct aberrations by placing a plurality of
aberration correction elements one over the other in order to
process all of the terms of Zernike polynomials.
In order to correct the third-order spherical aberration and the
fifth-order spherical aberration, it is necessary to create a phase
modulation profile from two phase distribution patterns
corresponding to respective aberrations. FIG. 3A illustrates a
curve 300 representing a phase distribution of third-order
spherical aberration, and FIG. 3B illustrates a curve 301
representing a phase distribution of fifth-order spherical
aberration. The aberrations in this case have a point-symmetric
phase distribution. Each of the curves illustrates a sectional view
of the phase distribution. Further, the vertical axis indicates a
value obtained by normalizing the phase difference, setting that
the positive maximum value of the phase difference is "1", and the
horizontal axis indicates a value obtained by normalizing the
effective diameter, setting that the maximum value of the effective
diameter is "1". In other words, the position "0" on the horizontal
axis corresponds to a position on the optical axis.
It is believed that the phase distribution of actually generated
aberrations is a linear sum of these aberrations. In view of the
above, a phase distribution is obtained by adding an adequate phase
distribution component resulting from defocus to the phase
distribution of a spherical aberration component which is the sum
of a third-order spherical aberration component and a fifth-order
spherical aberration component. Then, a profile, whose polarity is
opposite to the polarity of the obtained phase distribution and
which cancels the phase distribution, is defined as a phase
modulation profile. For instance, when an objective lens has a
numerical aperture NA of 1.0, the ratio between generated
third-order spherical aberration and generated fifth-order
spherical aberration is about 4:1, and it is possible to define a
profile, whose polarity is opposite to the polarity of a phase
distribution obtained by adding a phase component resulting from
defocus to the linear sum of these spherical aberrations, as a
phase modulation profile.
As described above, in correcting aberrations by a correction ring,
it is necessary to repeat adjustment of the correction ring and
focusing, which makes the optimization process long and
complicated. However, the idea of correcting a phase distribution
(a defocus component) that results from focusing as a phase
modulation profile by the phase modulation device 3 makes it
possible to eliminate the repeating process for optimization, and
to efficiently correct aberrations.
Further, the aberration components to be corrected by the phase
modulation device 3 are not limited to defocus and spherical
aberrations. It is possible to correct various generated
aberrations, for instance, still higher-order aberrations, or
aberrations that are not spherically symmetrical such as coma
aberration. Each of the aberrations has an amount that enables to
cancel the aberrations each other. Therefore, the total phase
modulation amount in complex aberrations, which is the sum of
aberrations of n kinds, is not equal to n-multiple of each of the
aberration correction amounts. Thus, using the complex aberrations
as aberrations to be corrected is advantageous because it is only
necessary for the phase modulation device 3 to impart a light flux
with a modulation amount sufficiently smaller than the total phase
modulation amount in the complex aberrations.
Next, a phase modulation profile for use in actually correcting
aberrations by the phase modulation device 3 is described in
details by an example. It is conceived that a phase distribution
that remains by focusing matches with a shape such that the root
mean square (RMS) value of the wave front having the phase
distribution is minimum. Therefore, for instance, there is proposed
a method, in which a phase distribution of complex aberrations
including a defocus term is obtained in such a manner that the RMS
aberration is minimized, and a phase modulation profile is defined
from the phase distribution.
A curve 400 illustrated in FIG. 4 represents a phase distribution
of complex aberrations in which a defocus component and a spherical
aberration generated on an objective lens whose numerical aperture
NA is 1.0 are added in such a manner that the RMS aberration is
minimized.
Further, there is also proposed an approach, in which a defocus
component is added so that the phase modulation amount
(hereinafter, called as a PV value) of a phase distribution is
minimized, and a phase distribution corresponding to the minimum
phase modulation amount is defined as a phase modulation profile. A
curve 500 illustrated in FIG. 5 represents a phase distribution of
complex aberrations when a defocus component is added in such a
manner that the PV value is minimized. When the PV value is
minimized, it is possible to set the phase modulation range (i.e.
range of the phase modulation amount) to be small. Therefore, when
a liquid crystal element is used as a phase modulation element in
the phase modulation device, it is possible to make the thickness
of the liquid crystal layer of the liquid crystal element to be
small. Further, generally, a response time of a liquid crystal
element is proportional to a square of the thickness of a liquid
crystal layer. Therefore, the smaller the phase modulation range
is, the higher the response speed is. Further, the smaller the
thickness of the liquid crystal layer is, the more the surface
precision is.
Further, it is believed that the phase distribution that remains by
focusing differs depending on the specifications of the microscope
for use or the image processing software for use. It is possible to
optimize the aberration correction by combining a residual
aberration pattern specific to each of the microscope and the image
processing software with the phase modulation profile of the phase
modulation device.
Next, the phase modulation device 3 configured such that a liquid
crystal element is used as a phase modulation element, and a
voltage is applied to each of electrodes of the liquid crystal
element, while using a phase distribution that cancels wave front
aberration as a phase modulation profile is described in detail,
referring to FIGS. 6 to 9A, and 9B.
FIG. 6 is a plan view of a phase modulation element 11 in the phase
modulation device 3. A liquid crystal layer of the phase modulation
element 11 is sandwiched between transparent substrates 21 and 22,
and the periphery of the liquid crystal layer is sealed by a
sealing member 23 so as to prevent leakage of liquid crystal. A
plurality of transparent annular electrodes are formed in
concentric form, the center of which is the optical axis, in an
active region 24 that drives the liquid crystal, in other words, in
a region capable of modulating the phase of a transmitting light
flux, on surfaces of the transparent substrates 21 and 22 disposed
to face each other. A transparent electrode may be formed on one of
the transparent substrates 21 and 22 in such a manner as to cover
the entirety of the active region 24. The active region 24 has a
size determined in accordance with the pupil diameter of the
objective lens. Controlling the voltages to be applied to the
transparent annular electrodes by a control circuit 12 in the phase
modulation device 3 makes it possible to impart a light flux
transmitting through the phase modulation element 11 with an
intended phase distribution. The control circuit 12 includes, for
instance, a processor, and a drive circuit capable of changing the
voltages to be output in accordance with a drive signal from the
processor.
FIG. 7 is a sectional schematic view of a part of the active region
24 of the phase modulation element 11 in FIG. 6. The phase
modulation element 11 is configured such that liquid crystal
molecules 34 are sandwiched between the transparent substrates 21
and 22. Transparent electrodes 33, 33a, and 33b are formed on the
surfaces of the transparent substrates 21 and 22 disposed to face
each other. FIG. 7 illustrates a state that a voltage is applied
between the electrode 33a on the right half side and the electrode
33, and a voltage is not applied between the electrode 33b on the
left half side and the electrode 33. The liquid crystal molecules
34 have an elongated molecular structure, and are homogeneously
aligned. Specifically, the liquid crystal molecules 34 sandwiched
between the two substrates 21 and 22 are aligned to be parallel to
each other in the major axis direction thereof, and are aligned in
parallel to each interface between each of the substrates 21 and 22
and the liquid crystal layer. In the liquid crystal molecules 34,
the refractive index thereof in the major axis direction and the
refractive index thereof in a direction orthogonal to the major
axis direction differ from each other. Generally, the refractive
index n.sub.e with respect to a polarized component (extraordinary
ray) in parallel to the major axis direction of the liquid crystal
molecules 34 is higher than the refractive index n.sub.o with
respect to a polarized component (ordinary ray) in parallel to the
minor axis direction of the liquid crystal molecules. Therefore,
the phase modulation element 11 configured such that the liquid
crystal molecules 34 are homogeneously aligned acts as a uni-axial
birefringent element.
Liquid crystal molecules have a dielectric anisotropy, and
generally, a force is exerted on the liquid crystal molecules such
that the major axis of the liquid crystal molecules is aligned with
the electric field direction. In other words, as illustrated in
FIG. 7, when a voltage is applied between the electrodes provided
in the two substrates for sandwiching the liquid crystal molecules
therebetween, the major axis direction of the liquid crystal
molecules is inclined from a state in parallel to the substrates
toward a direction orthogonal to the surfaces of the substrates in
accordance with the voltage. The refractive index n.sub..phi. of
the liquid crystal molecules with respect to a light flux of a
polarized component in parallel to the major axis of the liquid
crystal molecules is represented by:
n.sub.o.ltoreq.n.sub..phi..ltoreq.n.sub.e (where n.sub.o is the
refractive index of ordinary light, and n.sub.e is the refractive
index of extraordinary light). Therefore, assuming that the
thickness of the liquid crystal layer is d, an optical path length
difference .DELTA.nd(=n.sub..phi.d-n.sub.od) is generated between
the light flux passing through a region where a voltage is applied,
and the light flux passing through a region where a voltage is not
applied in the liquid crystal layer. The phase difference is
2.pi..DELTA.nd/.lamda., where .lamda. is the wavelength of a light
flux incident on the liquid crystal layer.
Next, a method for imparting a light flux transmitting through the
phase modulation element 11 as a liquid crystal element with an
intended phase distribution is described in detail. First of all, a
phase modulation profile to be displayed is determined, and a
voltage to be applied to each of the annular electrodes is
determined by dividing the phase modulation profile at a fixed
phase interval.
FIG. 8 is a diagram illustrating a manner as to how a voltage
application state is determined in accordance with a phase
modulation profile. A curve 800 on the upper side in FIG. 8
represents a sectional view of a phase modulation profile
corresponding to a plane passing the optical axis. On the lower
side of FIG. 8, there are illustrated annular electrodes 810, for
each of which an applied voltage value is determined in accordance
with the phase modulation profile. The bold lines in FIG. 8
illustrate spaces between the annular electrodes. Lead-out
electrodes and other elements are not illustrated to simplify the
illustration. Applying a voltage to each of the annular electrodes
by the control circuit 12 in such a manner that a voltage
difference between the adjacent annular electrodes corresponds to a
fixed step in a voltage range, in which characteristics of the
phase modulation amount to be imparted to a light flux transmitting
through the phase modulation element 11 with respect to the applied
voltage is substantially linear, allows for the phase modulation
device 3 to display a profile, in which an intended phase
distribution is quantized.
In order to apply a voltage to each of the annular electrodes in
such a manner that the voltage difference between the adjacent
annular electrodes corresponds to a fixed step, the annular
electrodes corresponding to the position where the phase modulation
amount is maximum and corresponding to the position where the phase
modulation amount is minimum are determined from the phase
modulation profile. The control circuit 12 applies an applied
voltage serving as the maximum phase modulation amount, and an
applied voltage serving as the minimum phase modulation amount to
the corresponding annular electrodes, respectively. Further, the
annular electrodes adjacent to each other are connected by an
electrode (a resistor) having a fixed electrical resistance.
Therefore, the voltage difference between the annular electrodes
adjacent to each other corresponds to a fixed step by resistance
division. Further, controlling the applied voltages as described
above is advantageous in simplifying the configuration of the
control circuit 12, as compared with a circuit configured to
control voltages to be applied to the annular electrodes
independently of each other.
FIGS. 9A and 9B are diagrams illustrating a relationship between
annular electrodes and applied voltages to be applied thereto when
the phase modulation device 3 has n annular electrodes. The center
electrode is called as the first annular electrode, the outermost
peripheral annular electrode is called as the n-th annular
electrode, and the annular electrode to which a maximum voltage is
applied is called as the m-th annular electrode.
FIG. 9A illustrates annular electrodes to which voltages are
applied by the control circuit 12 by 2-level driving. A voltage V1
is applied to the first annular electrode at the center and to the
n-th annular electrode at the outermost periphery, and a voltage V2
is applied to the m-th annular electrode. Selecting a defocus value
in such a manner that the phase modulation amounts at the center
and at the end in a phase distribution of generated wave front
aberrations are equal to each other makes it possible to match the
phase modulation amount at the center electrode with the phase
modulation amount at the outermost peripheral electrode. As a
result of the above control, the voltage applied to the center
electrode is made equal to the voltage value of the voltage to be
applied to the n-th annular electrode at the outermost periphery.
Further, applying the voltages as described above makes it possible
to minimize the PV value. In this way, in the example of 2-level
driving, it is possible to vary the amplitude of the phase
modulation amount without changing the relative ratio of the phase
modulation profile, with use of a difference between the applied
voltages V1 and V2. Further, the above driving method has a feature
that the phase modulation profile has a fixed shape, regardless of
the advantage that the number of levels of the voltage values to be
directly applied to the annular electrodes by the control circuit
12 are only two.
On the other hand, FIG. 9B illustrates annular electrodes to which
voltages are applied by the control circuit by 3-level driving. The
method in FIG. 9B is substantially the same as the method for
determining the voltage to be applied to each of the annular
electrodes by 2-level driving in a point that the voltage to be
applied to each of the annular electrodes is determined by
resistance division. In the configuration of FIG. 9B, however, a
voltage V3 to be applied to the n-th annular electrode at the
outermost periphery by the control circuit 12 may be different from
a voltage V1 to be applied to the first annular electrode. In this
way, applying the voltages to the center electrode and to the
outermost peripheral electrode independently of each other in such
a manner that an intended phase modulation amount is generated with
respect to the n-th annular electrode at the outermost periphery
makes it possible to compensate aberrations with high precision by
causing the phase modulation device 3 to display a phase modulation
profile in accordance with a numerical aperture NA, even when
objective lenses having numerical apertures NAs different from each
other are used. In this way, using the 3-level driving makes it
possible to extend the degree of freedom with respect to a phase
modulation profile to be displayed on the phase modulation element
11, and as described above, allows for the phase modulation device
3 to display a phase modulation profile in accordance with
objective lenses having a fixed pupil diameter but having numerical
apertures NAs different from each other. Further, using the 3-level
driving makes it possible to flexibly change the shape of a phase
modulation profile such as approximation of a phase modulation
profile to a profile corresponding to a different pupil diameter at
a fixed numerical aperture NA. Thus, the phase modulation device 3
is operable to compensate aberrations, while suppressing residual
aberrations.
Next, a method for varying a phase modulation profile to be
displayed by the phase modulation device 3 in accordance with
objective lenses having numerical apertures NAs different from each
other is described in detail.
First of all, quantitative determination on spherical aberrations
generated by numerical apertures NAs is made in order to obtain a
phase modulation profile optimum for each of the numerical
apertures NAs. The ratio between third-order spherical aberration
coefficient Z.sub.13 and fifth-order spherical aberration
coefficient Z.sub.25 when the numerical aperture NA is 1.2 is about
2.4:1.
FIG. 10A is a diagram illustrating a relationship between the
numerical aperture NA and the ratio (Z.sub.25/Z.sub.13) between
Z.sub.13 and Z.sub.25 when the numerical aperture NA of the
objective lens in the laser microscope is within the range of from
1.15 to 1.27, and the objective lens is an immersion lens. A
straight line 1000 is an approximation straight line representing a
relationship between the numerical aperture NA and the ratio
(Z.sub.25/Z.sub.13). Further, FIG. 10B is a diagram illustrating a
relationship between the numerical aperture NA and the ratio
(Z.sub.25/Z.sub.13) between Z.sub.13 and Z.sub.25 when the
numerical aperture NA of the objective lens in the laser microscope
is within the range of from 1.05 to 1.27. A straight line 1001 is
an approximation straight line representing a relationship between
the numerical aperture NA and the ratio (Z.sub.25/Z.sub.13).
Further, FIG. 10C is a diagram illustrating a relationship between
the numerical aperture NA and the ratio (Z.sub.25/Z.sub.13) between
Z.sub.13 and Z.sub.25 when the numerical aperture NA of the
objective lens in the laser microscope is within the range of from
0.75 to 0.95, and the objective lens is a dry lens. A straight line
1002 is an approximation straight line representing a relationship
between the numerical aperture NA and the ratio
(Z.sub.25/Z.sub.13). In FIGS. 10A to 10C, the horizontal axis
indicates the numerical aperture NA, and the vertical axis
indicates the ratio (Z.sub.25/Z.sub.13).
The curves 1000 to 1002 are respectively represented by the
following equations (1) to (3).
.times..times..times. ##EQU00004##
Preferably, the phase distribution to compensate the generated
spherical aberrations may be a linear sum of aberrations
represented by the ratios as described above.
A Strehl ratio, as one of the indexes indicating the performance of
an optical imaging system is known. The Strehl ratio is a ratio
between a peak luminance, on an imaging surface, of light from a
point light source in an optical system, and a peak luminance in a
diffraction limit optical system. An optical system having a Strehl
ratio closer to 1 is regarded as an optical system having a higher
imaging performance. Generally, as far as the Strehl ratio is 0.8
or larger, it is possible to neglect the influence on the imaging
performance due to residual aberrations (see e.g. page 198
"Introduction to Optics for User Engineers" by Toshiro KISHIKAWA,
published by the Optronics Co., Ltd.). The Strehl ratio and the
wave front aberration (.sigma., RMS value) have a relationship as
represented by the equation (4). Further, the relationship between
the wave front aberration (RMS value) and each of the wave front
aberration coefficients is represented by the equation (5). In this
example, the wave front aberration coefficients are values in the
unit of rad.
.function..sigma..apprxeq..sigma..sigma..times..times..sigma..times..time-
s. ##EQU00005##
Therefore, assuming that third-order spherical aberration A.sub.13
is generated in an immersion objective lens whose numerical
aperture NA is 1.2, for instance, it is conceived that fifth-order
spherical aberration is generated by 0.45A.sub.13. In this case, if
aberration is corrected in a state that the value of fifth-order
spherical aberration is shifted by x, (xA.sub.13)/(7.sup.1/2) (RMS
value) remains as fifth-order aberration according to the equation
(5).
As described above, as far as the Strehl ratio is 0.8 or larger, it
is possible to reduce the influence on the imaging performance due
to residual aberrations. Therefore, assuming that the tolerance of
the Strehl ratio is 0.8, the maximum value of the shift amount x is
represented by the equation (6). Therefore, taking into
consideration a difference in numerical aperture NA, preferably,
the ratio (A.sub.25/A.sub.13) may satisfy the relationship
represented by the following equation.
.times..ltoreq..ltoreq..times. ##EQU00006##
.times..times..times..times..times..ltoreq..ltoreq. ##EQU00006.2##
.times..ltoreq..ltoreq..times. ##EQU00006.3##
.times..times..times..times..times..ltoreq..ltoreq. ##EQU00006.4##
.times..ltoreq..ltoreq..times. ##EQU00006.5##
.times..times..times..times..times..ltoreq..ltoreq.
##EQU00006.6##
Let us study an example of the range of x that satisfies the Strehl
ratio of 0.8 in a condition that a specimen is observable
regardless of the presence or absence of a cover glass.
Third-order spherical aberration coefficient of spherical
aberration generated by the presence or absence of a cover glass is
about 5.2, taking into consideration that the thickness of a
general cover glass is 0.17 mm. It is preferable for the phase
modulation device 3 to correct the aforementioned aberrations from
a neutral position. Therefore, a correction amount having a range
of about 10 is necessary. The value of x is a little less than 0.23
when the wavelength is 488 nm, for instance, according to the
equation (6) described above. The ratio of fifth-order spherical
aberration with respect to third-order spherical aberration is
preferably about 0.42.+-.0.23.
Assuming that the ratio between third-order spherical aberration
component and fifth-order spherical aberration component is A:B
when a phase distribution generated at the numerical aperture NA of
an objective lens is resolved using Zernike polynomials, the
relationship between the numerical aperture NA and the ratio B/A
can be represented by the following equation.
.times..ltoreq..ltoreq..times. ##EQU00007##
.times..times..times..times..times..ltoreq..ltoreq. ##EQU00007.2##
.times..ltoreq..ltoreq..times. ##EQU00007.3##
.times..times..times..times..times..ltoreq..ltoreq. ##EQU00007.4##
.times..ltoreq..ltoreq..times. ##EQU00007.5##
.times..times..times..times..times..ltoreq..ltoreq.
##EQU00007.6##
Further, actual objective lenses have a variety of pupil diameters
and numerical apertures NAs. Therefore, it is preferable to create
a phase modulation profile in accordance with a variety of
combinations of pupil diameters and numerical apertures NAs. In
this case, it is preferable to use one common configuration of
electrodes for driving a liquid crystal layer of the phase
modulation element 11, regardless of the difference in pupil
diameter and numerical aperture NA. It is preferable to insulate
the annular electrodes from each other so that the control circuit
12 is operable to apply an intended voltage to each of the annular
electrodes. According to this configuration, the control circuit 12
is operable to cause the phase modulation element 11 to display a
phase modulation profile in accordance with the pupil diameter and
the numerical aperture NA by controlling the voltages to be applied
to the annular electrodes independently of each other.
FIGS. 11A, 11B, and 11C illustrate a difference in phase
distribution of aberration due to a difference in numeral aperture
NA of the objective lens. A curve 1100 illustrated in FIG. 11A, a
curve 1101 illustrated in FIG. 11B, and a curve 1102 illustrated in
FIG. 11C respectively indicate phase distributions of complex
aberrations when the numerical aperture NA is 1.0, 1.2, and 0.8. In
FIGS. 11A to 11C, the vertical axis indicates a value obtained by
normalizing the phase difference, setting that the positive maximum
value of the phase difference is "1", and the horizontal axis
indicates a value obtained by normalizing the effective diameter,
setting that the maximum value of the effective diameter is
"1".
In this example, the defocus value is fixed with respect to each of
the numerical apertures NAs. When the phase distribution of
aberration differs depending on the objective lenses, the control
circuit 12 controls the voltage to be applied to each of the
annular electrodes by 3-level driving as illustrated in FIG. 9B so
that various phase modulation profiles are reproducible. The ratios
between the voltages V1, V2, and V3 to be applied to the first
annular electrode, the m-th annular electrode, and the n-th annular
electrode are obtained in advance in such a manner that the phase
distribution of aberration is cancelled, each time the objective
lens is exchanged. A storage unit in the control circuit 12 may
store in advance the ratios between the voltages in correspondence
to the pupil diameter and the numerical aperture NA of the
objective lens. Then, the control circuit 12 may read the ratios
between the voltages V1, V2, and V3 in accordance with the
objective lens from the storage unit, and may determine the voltage
to be applied to each of the annular electrodes in accordance with
the ratios. The final voltage adjustment (voltages V1, V2, and V3)
may be manually performed by allowing the user to view an image via
an unillustrated user interface. Alternatively, the control circuit
12 may automatically set the voltages that maximize the contrast,
while feedback controlling the information to be obtained from an
image such as contrast.
In the foregoing, examples of 2-level driving and 3-level driving
have been described. The invention, however, is not limited to the
above. For instance, although the number of voltage levels
increases, wiring may be provided for each of the annular
electrodes so that voltages different from each other are applied
to the individual annular electrodes. In this modification, even if
the objective lens is changed, the control circuit is operable to
cause the phase modulation element 11 to display a phase modulation
profile optimum for the objective lens. As a result, the laser
microscope 1 is operable to acquire a desirable image.
Further, as described above, a phase difference depends on the
wavelength of light to be incident on a liquid crystal layer. The
laser light source 1 in a general laser microscope is operable to
irradiate laser light of a selected wavelength from among a
plurality of wavelengths of laser light. However, a required phase
modulation amount differs depending on the wavelength of laser
light for use. Therefore, it is necessary for the control circuit
12 of the phase modulation device 3 to correct the phase modulation
amount due to the phase modulation element 11. The control circuit
is operable to correct a phase modulation amount deviation due to a
difference in wavelength by changing the voltage to be applied to
the liquid crystal layer of the phase modulation device 3. Further,
the control circuit 12 is operable to cancel a phase modulation
amount deviation due to a temperature difference or the like by
adjusting the voltage to be applied to the liquid crystal layer of
the phase modulation element 11.
In the following, a method for obtaining an optimum phase
modulation amount due to a difference in wavelength of laser light
is described. A curve 1200 illustrated in FIG. 12 indicates
wavelength dispersion characteristics of liquid crystal provided
and sealed in a liquid crystal layer of the phase modulation device
3 in the foregoing embodiment. The horizontal axis indicates a
wavelength, and the vertical axis indicates a value obtained by
normalizing the phase difference (And) of the phase modulation
device 3 in such a manner that the value of the phase difference
when the wavelength is 550 nm is equal to 1. As represented by the
curve 1200, for instance, the degree of wavelength dispersion is
1.057 when the wavelength of laser light is 488 nm, and the degree
of wavelength dispersion is 1.200 when the wavelength of laser
light is 405 nm. This reveals that .DELTA.n(=n.sub.e-n.sub.o)
differs depending on the wavelength of laser light, because the
thickness d of the liquid crystal layer has a fixed value.
Therefore, even if the specimen 5 illustrated in FIG. 1 is observed
at a fixed position, an optimum phase modulation profile differs
depending on the wavelength of the laser light source 1 for use. In
view of the above, it is preferable to optimize a phase modulation
profile by adding a degree of wavelength dispersion optimum for a
target wavelength, as a parameter, to the calculation equation
representing a phase modulation profile so that the phase
modulation element 11 imparts a transmitting light flux with the
optimum phase modulation profile.
Specifically, it is necessary to use the wavelength of the laser
light source 1 for use as a parameter in order to create a phase
modulation profile. In other words, multiplying a degree of
wavelength dispersion as illustrated in FIG. 12 as a coefficient by
the phase modulation profile created as described above makes it
possible to obtain an optimized phase modulation profile, taking
into consideration of the wavelength of laser light from the laser
light source. The control circuit 12 may adjust the voltage to be
applied to each of the electrodes of the phase modulation element
11, based on the optimized phase modulation profile.
Further, in the embodiments described above, a liquid crystal
element is used as the phase modulation element of the phase
modulation device, but the phase modulation element is not limited
to a liquid crystal element. For instance, an optical crystal
element having an electro-optical effect as represented by a
Pockels effect may be used as the phase modulation element. In this
modification, as well as the case of using a liquid crystal
element, annular electrodes the center of which is the optical axis
are mounted on one surface of an optical crystal element on a flat
plate, and an electrode is mounted on the other surface of the
optical crystal element so as to cover the entirety of the surface.
As well as the embodiments, each of the electrodes may preferably
be a transparent electrode. In this modification, as well as the
embodiments, adjusting the voltage applied to each of the annular
electrodes by the control circuit makes it possible to cause the
optical crystal element to display a phase modulation profile for
correcting aberrations of an optical system including an objective
lens, and to impart a light flux transmitting through the optical
crystal element with a phase distribution in accordance with the
phase modulation profile.
As another modification, a deformable mirror may be used as the
phase modulation element, although the deformable mirror is of a
reflective-type mirror. In this modification, annular electrodes,
the center of which is the optical axis, are mounted on the
deformable mirror. Adjusting the voltage to be applied to each of
the annular electrodes by the control circuit makes it possible to
represent a phase modulation profile that corrects aberrations of
an optical system including an objective lens by the deformable
mirror, and to impart a light flux reflected on the deformable
mirror with a phase distribution in accordance with the phase
modulation profile.
The embodiments have been described by an example of a laser
microscope. Use of the phase modulation device of the invention is
not limited to the above example. The invention may be applied to
any optical apparatus, as far as the optical apparatus is provided
with an objective lens and is incorporated with a coherent light
source. The invention is also applicable to an OCT (Optical
Coherence Tomography), for instance.
As is evident from the above description, those skilled in the art
can make various modifications to the embodiments without departing
from the scope and spirit of the present invention.
REFERENCE SIGNS LIST
1 laser light source 2 collimating optical system 3 phase
modulation device 4 objective lens 5 specimen 6 beam splitter 7
confocal optical system 8 confocal pinhole 9 detector 11 phase
modulation element 12 control circuit 21, 22 transparent substrate
23 sealing member 33 transparent electrode 34 liquid crystal
molecules
* * * * *
References